The present invention is directed to a system and method for detecting contact of an electrode with tissue. The system and method of the invention are particularly suited for use in conjunction with intracardiac electrophysiology or electromechanical studies or in conjunction with therapeutic procedures such as cardiac ablation.
Cardiac arrhythmias, the most common of which is ventricular tachycardia (VT), are a leading cause of death. In a majority of patients, VT originates from a 1 mm to 2 mm lesion located close to the inner surface of the heart chamber. One of the treatments for VT comprises mapping the electrical pathways of the heart to locate the lesion followed by ablation of the active site.
Commonly assigned U.S. Pat. No. 5,546,951; U.S. patent application Ser. No. 08/793,371; and PCT application WO 96/05768, which are incorporated herein in their entirety by reference, disclose methods for sensing an electrical property of heart tissue such as local activation time as a function of the precise location within the heart. The data are acquired by advancing into the heart one or more catheters that have electrical and location sensors in their distal tips. The precise three-dimensional location of the catheter tip is ascertained by the location sensor contained therein. The location sensor operates by generating signals that are responsive to its precise location within an externally generated non-ionizing field such as an electromagnetic field. Simultaneous with the acquisition of location information, electrical information is also acquired by at least one electrode contained at the catheter distal tip. Accurate sensing of location and electrical information by sensors contained in the catheter generally requires a high degree of confidence that a catheter electrode is in contact with the tissue.
In systems that use acoustic means to determine the location of mapping and ablation electrodes, it is likewise important to determine that the electrodes are in contact with the tissue to be mapped or ablated. For example, U.S. Pat. No. 5,409,000, the disclosure of which is incorporated herein in its entirety by reference, discloses the use of a catheter probe having a plurality of flexible, longitudinally extending circumferentially spaced apart arms adapted to be disposed within a chamber of a heart. Electrodes are carried by the arms and are adapted to be moved into engagement with the wall of the heart. Markers visible ultrasonically are carried by the arms for encoding the arms so that one arm can be distinguished from another. An ablation catheter having ultrasonic viewing means such as an ultrasonic sensor or transducer at its distal extremity is carried by and is slidably mounted in the catheter probe. The distal extremity of the ablation catheter is moveable into positions to view ultrasonically the markers carried by the arms of the catheter probe so that the arms can be identified and the spacing of the arms can be ascertained.
PCT application WO 99/05971, the disclosure of which is incorporated herein in its entirety by reference, discloses a system that uses one or more ultrasound reference catheters to establish a fixed, three-dimensional coordinate system within a patient""s heart using principles of triangulation. The coordinate system is represented graphically in three dimensions on a video monitor and is reported to aid the clinician in guiding other medical devices, which are provided with ultrasound sensors or transducers, through the body to locations at which they are needed to perform clinical procedures. The system is reported to be useful to help a physician guide mapping catheters for measuring electrical activity and ablation catheters for ablating selected regions of cardiac tissue, to desired locations within the heart.
Methods of creating a map of the electrical activity of the heart based on these data are disclosed in commonly assigned U.S. patent application Ser. Nos. 09/122,137 and 09/357,559 filed on Jul. 24, 1998 and Jul. 22, 1999, respectively, and in European Patent Application 974,936 which are also incorporated herein in their entirety by reference. In clinical settings, it is not uncommon to accumulate data at 100 or more sites within the heart to generate a detailed, comprehensive map of heart chamber electrical activity. The use of the location sensors as hereinabove described is highly useful in providing a detailed and accurate map of the heart chamber""s activity.
Catheters containing position or location sensors may also be used to determine the trajectory of points on the cardiac surface. These trajectories may be used to infer mechanical motion characteristics such as the contractility of the tissue. As disclosed in U.S. Pat. No. 5,738,096 which is incorporated herein in its entirety by reference, maps depicting such motion characteristics, which may be superimposed with maps depicting local electrical information, may be constructed when the trajectory information is sampled at a sufficient number of points in the heart. Accurate maps of such motion characteristics again require confidence that the data are acquired when the catheter tip is in contact with the cardiac tissue.
The detailed maps generated as hereinabove described may serve as the basis for deciding on a therapeutic course of action, for example, tissue ablation, to alter the propagation of the heart""s electrical activity and to restore normal heart rhythm. In cardiac ablation, energy, typically in the radiofrequency (RF) range, is supplied at selected points on the intracardiac surface by a catheter having an ablation electrode at its distal tip. Ablation is effected by bringing the distal tip electrode into contact with the locus of aberrant electrical activity and by initiating the delivery of RF energy through the distal tip electrode from an external RF generator in communication with the distal tip electrode. Ablation is most effectively performed when the distal tip electrode is in contact with the cardiac wall. Absence of contact or poor contact of the tip electrode with the heart wall leads to dissipation of the RF energy in the blood, as well as possible fouling of the tip electrode with the concomitant possibility of blood clot formation. Accordingly, it is important that both mapping and ablation be accompanied by methods and systems for detecting and ensuring electrode-tissue contact.
A number of references have reported methods to determine electrode-tissue contact, including U.S. Pat. Nos. 5,935,079; 5,891,095; 5,836,990; 5,836,874; 5,673,704; 5,662,108; 5,469,857; 5,447,529; 5,341,807; 5,078,714; and Canadian Patent Application 2,285,342. A number of these references, e.g., U.S. Pat. Nos. 5,935,079, 5,836,990, and 5,447,529 determine electrode-tissue contact by measuring the impedance between the tip electrode and a return electrode. As disclosed in the ""529 patent, it is generally known that impedance through blood is generally lower that impedance through tissue. Accordingly, tissue contact has been detected by comparing the impedance values across a set of electrodes to pre-measured impedance values when an electrode is known to be in contact with tissue and when it is known to be in contact only with blood. A problem in using this method during intracardiac procedures is the fact that tissue and blood impedances may change during a procedure. Furthermore, the impedance through tissue also depends on the state of the tissue. For instance, impedance through infarcted tissue is known to be less than the impedance through healthy tissue.
U.S. Pat. No. 5,341,807 discloses a method of detecting contact of a catheter tip electrode with tissue. The method of the ""807 patent employs a catheter having a tip electrode and a plurality of axially spaced ring electrodes mounted along the catheter surface. A test signal is applied across a pair of outer electrodes arranged along the catheter. Each outer electrode is paired with an inner electrode to develop a sensing signal characteristic of impedance for the tissue between the electrodes. One major drawback to the catheter and associated method disclosed in the ""807 patent is that it relies on tissue impedance measurement as the sole manner for determining the position and orientation of the catheter. Furthermore, if the catheter electrodes used in the impedance measurements are also used with an ECG device to collect body surface and intracardiac ECG signals, the impedance measuring components of the ""807 patent would require a separate ground relative to the ECG device, which complicates the circuitry.
The present invention is directed to a novel system and method for detecting electrode-tissue contact. The system of the invention comprises a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode, preferably positioned at the catheter distal tip, and a location sensor. The system of the invention further comprises a reference electrode and a contact detection circuit. The contact detection circuit comprises a signal generator for sending test signals to the contact electrode and to the reference electrode. The contact detection circuit further comprises a circuit to measure a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue.
In the system of the invention, the reference electrode is preferably positioned on the catheter comprising the contact electrode and the location sensor. The reference electrode is further preferably protected from making contact with tissue. In one embodiment, the reference electrode is protected from tissue contact by a membrane covering the electrode. The membrane permits contact of the reference electrode with blood but does not permit contact of the reference electrode with tissue. In another embodiment, the reference electrode is protected from making tissue contact by recessing the electrode relative to the catheter body.
The system of the invention preferably further comprises a return electrode, which functions as a sink for the test signals to the contact electrode and to the reference electrode. In some embodiments, the return electrode is adapted for positioning internal to the body. For example, the return electrode may be positioned on the catheter comprising the contact electrode and the location sensor. In other embodiments, the return electrode is adapted for contact with skin external to the body. The return electrode may be dedicated for measuring differential signals with the contact electrode and the reference electrode. The return electrode is preferably connected to isolated ground, preferably, to an electrocardiogram device isolated ground.
The location sensor contained in the catheter used in the system of the invention may be of any type known in the art, for example, acoustic, magnetic or electromagnetic location sensors. Preferably, the location sensor is an electromagnetic location sensor.
In one embodiment, the system of the invention comprises a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode positioned at the catheter distal tip, a location sensor and a reference electrode. The system of the invention further comprises a contact detection circuit. The contact detection circuit comprises a signal generator for sending test signals to the distal tip contact electrode and to the reference electrode. The contact detection circuit further comprises a circuit to measure a differential electrical response to the test signals, the differential electrical response being indicative of contact of the distal tip contact electrode with tissue. The system of the invention further comprises a return electrode which functions as a sink for the test signals to the distal tip contact electrode and to the reference electrode. The circuit to measure a differential electrical response to the test signals comprises a first differential amplifier and a second differential amplifier. The first differential amplifier is used to measure a first electrical difference signal between the distal tip electrode and the return electrode. The second differential amplifier is used to measure a second electrical difference signal between the reference electrode and the return electrode. The system of the invention preferably further comprises a third differential amplifier to measure an electrical difference signal between the first electrical difference signal and the second electrical difference signal.
The first differential amplifier preferably measures the voltage difference between the distal tip electrode and the return electrode. The second differential amplifier preferably measures the voltage difference between the reference electrode and the return electrode. The third differential amplifier preferably measures the voltage difference between the first amplifier and the second amplifier. The electrical difference signal measured by the third differential amplifier is preferably rectified by a synchronous detector.
The gains of the first amplifier and the second amplifier are preferably adjusted such that the ratio of the gain of the first amplifier to the gain of the second amplifier is proportional to the ratio of the tip electrode area to the reference electrode area. When so adjusted, the output of the third amplifier will be a null signal when both the tip electrode and the reference electrode are in blood and neither electrode is in contact with tissue.
In this embodiment of the system of the invention, the distal tip electrode is preferably supplied with a first constant current and the reference electrode is supplied with a second constant current, the first current being equal to the second current. The return electrode is preferably driven with a third constant current opposite in phase with the first constant current and the second current.
In another embodiment, the system of the invention for detecting electrode-tissue contact comprises a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode positioned at the catheter distal tip, a location sensor and a reference electrode. The system of the invention further comprises a contact detection circuit. The contact detection circuit comprises a signal generator for sending test signals to the distal tip contact electrode and to the reference electrode. The contact detection circuit further comprises a circuit to measure a differential electrical response to the test signals, the differential electrical response being indicative of contact of the distal tip contact electrode with tissue. The system further comprises a return electrode which functions as a sink for the test signals to the distal tip contact electrode and to the reference electrode. The circuit to measure a differential electrical response to the test signals comprises a bridge circuit comprising a first resistive element and a second resistive element. The first resistive element and the second resistive element each have a first side and a second side. The first side of the first resistive element is electrically connected with the first side of the second resistive element. The second side of the first resistive element is electrically connected with the reference electrode and the second side of the second resistive element is electrically connected with the distal tip contact electrode. The bridge circuit has a first input between the first resistive element and the second resistive element and a second input electrically connected to the return electrode. The bridge has a first output between the first resistive element and the reference electrode and a second output between the second resistive element and the distal tip contact electrode. The bridge outputs are preferably connected to a differential amplifier which measures a bridge output voltage indicative of contact of the distal tip contact electrode with tissue. The output of the differential amplifier is preferably rectified by a synchronous detector.
In one variation of this embodiment, the first resistive element is a first resistor and the second resistive element is a second resistor. The ratio of the resistance of the first resistor to the resistance of the second resistor is preferably proportional to the ratio of the tip electrode area to the reference electrode area.
In another variation on this embodiment, the first resistive element is a first high output impedance buffer and the second resistive element is a second high output impedance buffer. The ratio of the output currents of the first high output impedance buffer to the second high output impedance buffer is preferably proportional to the ratio of the tip electrode area to the reference electrode area.
Another embodiment of the system of the invention for detecting electrode-tissue contact comprises a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode positioned at the catheter distal tip, a location sensor and a reference electrode. The system of the invention further comprises a contact detection circuit. The contact detection circuit comprises a signal generator for sending test signals to the distal tip contact electrode and to the reference electrode. The contact detection circuit further comprises a circuit to measure a differential electrical response to the test signals, the differential electrical response being indicative of contact of the distal tip contact electrode with tissue. The system further comprises a return electrode which functions as a sink for the test signals to the distal tip contact electrode and to the reference electrode. The circuit to measure a differential electrical response to the test signals comprises a first current sensor for measuring the current to the reference electrode and a second current sensor for measuring the current to the distal tip electrode. The current sensors are preferably selected from current transformers and Hall effect sensors. The ratio of the gain of the first current sensor to the gain of the second current sensor is preferably proportional to the ratio of the tip electrode area to the reference electrode area. The current sensors preferably have outputs connected to a differential amplifier that measures a voltage indicative of contact of the distal tip electrode with tissue. The differential amplifier preferably has an output rectified by a synchronous detector.
Another aspect of the invention is directed to a method for detecting electrode-tissue contact. The method of the invention comprises providing a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode adapted for contact with tissue, preferably positioned at the catheter distal tip, and a location sensor. The method of the invention further comprises providing a reference electrode, which is preferably positioned on the catheter comprising the contact electrode and the location sensor. The method of the invention further comprises the steps of providing test signals to the contact electrode and to the reference electrode, and measuring a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue.
The location sensor contained in the catheter used in the method of the invention is preferably an electromagnetic sensor.
In practicing the method of the invention, the reference electrode is preferably protected from making contact with tissue. In one embodiment, the reference electrode is protected from making tissue contact by a membrane covering the reference electrode; the membrane permitting contact of the reference electrode with blood but not permitting contact of the reference electrode with tissue. Alternatively, the reference electrode may be protected from making tissue contact by being recessed relative to the catheter body.
In one embodiment, the method of the invention comprises providing a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode adapted for contact with tissue positioned at the catheter distal tip, a location sensor and a reference electrode. The method of the invention further comprises the steps of providing test signals to the distal tip contact electrode and to the reference electrode, and measuring a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue. The measurement of the differential electrical response to the test signals comprises the steps of measuring a first electrical difference signal between the distal tip contact electrode and a return electrode; measuring a second electrical difference signal between the reference electrode and the return electrode; and comparing the first electrical difference signal with the second electrical difference signal to detect contact of the distal tip contact electrode with tissue.
In this embodiment of the method of the invention, the signals provided to the distal tip and reference electrodes are preferably constant current signals.
The comparison of the first and second electrical difference signals preferably comprises feeding the signals to a differential amplifier to produce a third electrical difference signal indicative of electrode-tissue contact. The first and second electrical difference signals are preferably adjusted to provide a null difference signal from the differential amplifier when the distal tip electrode and the reference electrode are both in blood and neither electrode is in contact with tissue.
In another embodiment, the method of the invention for detecting electrode-tissue contact comprises providing a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode adapted for contact with tissue positioned at the catheter distal tip, a location sensor and a reference electrode. The method of the invention further comprises the steps of providing test signals to the distal tip contact electrode and to the reference electrode, and measuring a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue. The measurement of the differential electrical response to the test signals comprises the steps of providing a bridge circuit comprising a first resistive element and a second resistive element. The first resistive element and the second resistive element each have a first side and a second side. The first side of the first resistive element is electrically connected to the first side of the second resistive element. The second side of the first resistive element is electrically connected with the reference electrode and the second side of the second resistive element is electrically connected with the distal tip electrode. The bridge circuit has a first input between the first resistive element and the second resistive element and a second input electrically connected to a return electrode. The bridge circuit further has a first output between the first resistive element and the reference electrode and a second output between the second resistive element and the distal tip electrode. The method of the invention further comprises measuring a signal across the bridge outputs to detect contact of the distal tip electrode with tissue. The signal across the bridge outputs is preferably measured with a differential amplifier, and is preferably adjusted to provide a null signal when the distal tip electrode and the reference electrode are both in blood and neither electrode is in contact with tissue.
In one variant of this embodiment of the method of the invention, the first resistive element comprises a first resistor and the second resistive element comprises a second resistor. In another variant, the first resistive element comprises a first high output impedance buffer and the second resistive element comprises a second high output impedance buffer.
In another embodiment, the method of the invention for detecting electrode-tissue contact comprises providing a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode adapted for contact with tissue positioned at the catheter distal tip, a location sensor and a reference electrode. The method of the invention further comprises the steps of providing test signals to the distal tip contact electrode and to the reference electrode, and measuring a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue. The measurement of the differential electrical response to the test signals comprises the steps of measuring current to the reference electrode with a first current sensor and measuring current to the distal tip electrode with a second current sensor. The outputs of the first current sensor and the second current sensor are connected to a differential amplifier to measure a differential voltage indicative of contact of the distal tip contact electrode with tissue. The current sensors are preferably of the current transformer or Hall effect type. The current sensors preferably have outputs connected to a differential amplifier that measures a voltage indicative of contact of the distal tip electrode with tissue. The signals from the current sensors are preferably adjusted to provide a null signal from the differential amplifier when the distal tip electrode and the reference electrode are both in blood and neither electrode is in contact with tissue.
In another embodiment, the method of the invention for detecting electrode-tissue contact comprises providing a catheter comprising a body having a proximal end and a distal end, the distal end having a distal tip. The catheter further comprises a contact electrode adapted for contact with tissue positioned at the catheter distal tip, a location sensor and a reference electrode. The method of the invention further comprises the steps of providing test signals to the distal tip contact electrode and to the reference electrode, and measuring a differential electrical response to the test signals, the differential electrical response being indicative of contact of the contact electrode with tissue. The measurement of the differential electrical response to the test signals comprises the steps of measuring a first impedance between the distal tip electrode and a return electrode and a measuring a second impedance between the reference electrode and a return electrode. The first and second impedances are compared to detect contact of the catheter distal tip contact electrode with tissue.
The method of the invention optionally further comprises collecting electrical information from the contact electrode, preferably positioned at the catheter distal tip, and location information from the location sensor at a plurality of points on the tissue. An electrical map of the tissue is then generated from the electrical and location information. The electrical and location information at each point in the map is weighted in accordance with contact being detected between the contact electrode and the tissue at each point.
The method of the invention optionally further comprises collecting electrical information from the contact electrode, preferably positioned at the catheter distal tip, and mechanical information from the location sensor, respectively at a plurality of points on the tissue. An electromechanical map of the tissue is then generated from the electrical and mechanical information. The electrical and mechanical information at each point in the map is weighted in accordance with contact being detected between the contact electrode and the tissue at each point.
The method of the invention optionally further comprises delivering ablation energy to the contact electrode, preferably positioned at the catheter distal tip, in accordance with the electrode being in contact with tissue.
It is an object of the invention to provide a system and method for detecting contact of an electrode with tissue.
It is another object of the invention to provide a differential system and method for detecting electrode-tissue contact with a contact electrode in comparison with a reference electrode.
It is another object of the invention to provide a differential system and method for detecting electrode-tissue contact with a contact electrode in comparison with a reference electrode not in contact with tissue.
It is another object of the invention to provide a system and method for detecting electrode-tissue contact in a system comprising a highly accurate location sensor.
It is another object of the invention to provide a system and method for detecting electrode-tissue contact for use in cardiac mapping procedures.
It is another object of the invention to provide a system and method for detecting electrode-tissue contact for use in cardiac ablation procedures.